The present invention relates to battery chargers and more particularly to a transformerless, battery-controlled, battery charger system intended to be carried on-board electrically-operated vehicles such as electric cars, golf carts, or light-weight industrial vehicles.
Conventional charging systems require the use of a power transformer. The conventional power transformer usually functions to isolate the DC output of a battery charger and the charged batteries from the AC power line, and/or reduces the AC line voltage to a value compatible with a vehicle battery voltage. In general, the transformer operates at the line frequency (50-60 Hz); however, in some recent designs transformers have operated at higher frequencies, typically several kilohertz, with the aid of switching-transistor choppers. This higher frequency greatly reduces the required size and weight for the transformer.
There is no absolute requirement that the battery of an electric vehicle cannot be connected directly to the AC line during charging, as is done in the present invention. Generally the electrical isolation of a power transformer is a welcome safety feature which protects the user in case he should accidentally come into simultaneous contact with the vehicle electrical system and an external ground. However, adequate protection can be provided in other ways: by insulating the vehicle electrical system; by grounding the vehicle frame with a central wire in the charging system power cables, and by including a ground fault interrupter at the charger AC line input. All three protective means are included in the present battery-charging system, and at least two, and generally all three, must fail before a user hazard can exist.
The present invention is for a transformerless, battery-charging system having three primary advantages over prior type charging systems, namely: the present system is light in weight, permitting the charging system to be carried on the vehicle with the attendant convenience of being able to recharge from any conventional AC electrical outlet; it operates at high energy efficiency, which benefits the user by reduced power consumption and cooler operation; and, it is relatively inexpensive. These advantages accrue mainly from the elimination of the power transformer required with conventional charging systems. The present battery charger is controlled from data received from all cells of the battery, providing battery-cell-controlled charging. The electrolyte levels, temperature, specific gravity and voltage are all analyzed to assure that each cell is fully charged. In the present system a switching regulator stores only a fraction of the energy in each charging pulse, and much of the charging energy is supplied directly from the powerline and never stored in the charger. Major components include an overload and ground fault protective current interruptor, an RF interference filter and line rectifier, and a switching regulator capable of maintaining a constant-current constant-voltage output, providing controlled input current throughout the AC cycle and including means for external shutdown.
Reduction of the AC line voltage to the battery charging voltage can be accomplished much more efficiently with a switching regulator than by a transformer, and this is one technique employed in the present invention. The switching regulator does require a ferromagnetic component, an inductor, but this component is much smaller and lighter than the power transformer of even a chopper-driven charger. The inductor of the switching regulator is required to store only a fraction of the energy in each charging pulse; much of the charging energy is supplied directly from the powerline source and never stored in the charger. This situation is directly analogous to the smaller size and weight of an autotransformer as compared to a conventional transformer supplying the same load. Not only is the magnetic component of the switching regulator smaller and lighter than the alternative transformer, the switching regulator can be made more efficient than a step-down transformer by varying its duty cycle through the AC line cycle in such a way as to control the input line current. Very high and quite unmanageable current pulses result from the direct application of unfiltered, rectified AC to a constant voltage load such as a storage battery.